In general, it has been known that a luminous flux of a discharge lamp such as a fluorescent lamp for use in an illumination purpose is decreased following elapse of an operating time. Accordingly, heretofore, a discharge lamp lighting device that suppresses the decrease of the luminous flux (illuminance) of the discharge lamp, which follows the elapse of the operating time, has been provided (for example, refer to Japanese Patent Laid-Open Publications Nos. H9-97683 and 2001-15275).
Such a conventional discharge lamp lighting device suppresses an illuminance of the discharge lamp at an initial period of such an operation at approximately 70 to 80% of a value at the time of rated lighting. The discharge lamp lighting device performs a light adjustment control so as to increase the luminous flux following the elapse of the operating time, and thereby can substantially constantly maintain an apparent illuminance during an operating period until the end of a lifetime. Moreover, the discharge lamp lighting device has an advantage that electric power saving can be achieved in such a manner that, at the initial period of the operation, the discharge lamp lighting device performs the light adjustment and the lighting while reducing electric power consumed in the discharge lamp than rated electric power.
Incidentally, a rated value of the electric power supplied to the discharge lamp (for example, the fluorescent lamp) is defined by a manufacturer of the discharge lamp. The electric power supplied to the discharge lamp is increased in substantially proportional to a shape of the discharge lamp. For example, as straight tube fluorescent lamps, there are: one with a shape in which a tube length is approximately 600 mm and a tube diameter is approximately 25.5 mm, and with defined electric power of approximately 23 W; and one with a shape in which a tube length is approximately 1200 mm and a tube diameter is approximately 25.5 mm, and with defined electric power of approximately 45 W. Moreover, as luminous flux characteristics of the fluorescent lamp when the rated electric power is supplied thereto, an initial luminous flux (luminous flux after elapse of 100 hours from the beginning of the operation) is made public. Furthermore, a rated lifetime of the discharge lamp is defined as a shorter one between a time until which a luminous flux deterioration reaches 70% of such initial characteristics and a time until which a filament is broken with regard to the straight tube fluorescent lamp.
Moreover, in indoor light equipment, in order to obtain an illuminance required for a working surface, the number of installed light fixtures (units) is decided by taking, as variables, a floor space, a lamp luminous flux (the above-described initial luminous flux), the number of lamps, a light loss factor, and the like. This light loss factor is decided so as to obtain the required illuminance at the point of time when the rated lifetime comes while considering the above-described luminous flux deterioration of the fluorescent lamp itself, a decrease of reflectivity owing to adhesion of dirt to the fluorescent lamp and the light fixture, and the like. For example, in a light fixture of a bottom-open type, a light loss factor thereof is set at 0.7 in a general operating environment, and the light loss factor is set at 0.74 as a guide in a place where an occurrence of dust is small and the air is always kept clean. In this light equipment, as the light loss factor becomes smaller, the number of light fixtures required to be installed is increased, leading to an increase of cost of the equipment. Hence, if it is assumed that types of the fluorescent lamps for use are the same, and that environments where the light fixtures are installed are the same, then illuminances at the time when the lifetime of the lamp is ended become substantially equal to each other between the following cases, which are: a case of using the general discharge lamp lighting device in which the illuminance is not suppressed from the initial period of the operation; and a case of performing the control to substantially constantly maintain the illuminance so that the illuminance can be always 70 to 80% of the value of the initial luminous flux while considering the decrease of the illuminance in advance. Then, the installed light fixtures in both of the cases also become equal in number to each other.
However, in comparison with a configuration of the general discharge lamp lighting device that does not suppress the illuminance from the initial period of the operation, it is necessary to add a configuration for performing the control to substantially constantly maintain the illuminance to the above-described conventional discharge lamp lighting device. Accordingly, cost of the device and the light fixture is increased, which in turn results in an increase of cost of a capital investment by a user.
Moreover, in the above-described discharge lamp lighting device, the illuminance at the initial period of the operation is suppressed to approximately 70 to 80%, whereby electric power saving of approximately 10 to 15% can be achieved during the period from the initial period of the operation to the rated lifetime in comparison with the general discharge lamp lighting device that does not suppress the illuminance from the initial period of the operation, thus making it possible to reduce running cost charged to the user. However, for the conventional discharge lamp lighting device, the electric power saving and resource saving cannot be desired any more in terms of obtaining the illuminance required for the working surface as a lighting subject of the light equipment.
The present invention has been made in consideration for the above-descried circumstances. It is an object of the present invention to provide a discharge lamp lighting device and a light fixture, which are capable of suppressing the cost of the initial investment by the user and the running cost to the user to a low extent while substantially constantly maintaining the apparent illuminance, and further, are capable of realizing the resource saving.